The present disclosure generally relates to sensors for use in liquid medication delivery systems and, in particular, to sensors for use in liquid medication delivery systems, with a micro-fluidic chamber and optical detection system, to infusion pump devices and liquid medicament delivery systems with such sensors, and the use of such sensors for measuring the pressure and/or the presence of air bubbles in a fluidic system.
Devices for the automated release of liquid medications are generally used with patients who have a continuous, and in the course of the day varying, need of a medicine that can be administered by subcutaneous infusion. Specific applications are, for example, certain pain therapies and the treatment of diabetes. In such cases, computer controlled infusion pumps are used, which can be carried by the patient on the body, and which contain a certain amount of liquid medication in a medicine reservoir. The medicine reservoir often comprises medicine sufficient for one or several days. The liquid medication is supplied to the patient from the medicine reservoir through an infusion cannula or an injection needle.
Particularly in self-administration of medications, for example insulin, the patients using the medication in question and administering it themselves by an infusion pump tend to emphasize convenience and discretion. As a consequence, the acceptable dimensions of such infusion pumps are limited in order not be evident through clothing and to be carried as comfortably as possible. In an advantageous type of infusion pump, the liquid medication is obtained by a downstream pump from a flexible container. Flexible containers have the advantage of a smaller volume surplus of the container in relation to its content, which reduces the manufacturing costs and enables design of infusion pumps of smaller overall dimensions.
In the context of liquid medication administration via an infusion pump, sensors can be used for controlling the dosing, monitoring the correct operation of the system, and for fast detection of faults and hazards, such as occluded infusion lines or cannulas, empty containers, or malfunctioning pump systems. A pressure sensor is typically arranged in the fluid path downstream of a pump and upstream of an infusion cannula.
Such pressure sensors typically comprise a micro-fluidic chamber filled with liquid and fluidly connected to the fluidic system. The micro-fluidic chamber is covered by a flexible, resilient membrane, such that a pressure difference between the fluidic pressure inside the sensor chamber and the outside (such as atmospheric) pressure will temporarily deform the membrane. The resulting deflection of the membrane can then be measured in order to determine the internal pressure of the fluidic system.
A suitable approach to measure the deformation of the membrane is optical detection of a light beam reflected off of the membrane. FIG. 1 schematically shows such a pressure sensor 6 according to the prior art. A micro-fluidic chamber 1 that is connected to a fluidic system comprises a rigid bottom substrate 11 and a flexible, resilient top cover 12, for example, a membrane. An optical detection system 5 is arranged to measure a deformation of the cover membrane 12 by determining the interaction of a light beam 53a with the cover membrane 12. For that purpose a light emitting device 51, e.g. a laser diode, directs a light beam 53a at a certain angle onto the surface of the cover membrane 12, where it is reflected 53b. The pressure difference Δp between the inner volume 14 of the micro-fluidic chamber 1 and the outer environment acts on the cover membrane 12, and deforms it to a certain extent 12′, depending on the pressure difference. As a result the angle of the reflected light beam changes and the beam 53a is transversely shifted. By observing the position of the reflected light beam 53b, 53b′, the deformation of the cover membrane 12 can be measured, and based on the obtained results a pressure difference value can be determined.
To observe the reflected light beam, a detector in an optical detection system 5 must be designed to be movable, or a multiplicity of detectors at different positions and at different angles must be included in a device according to the prior art. Both of these aspects make such sensor devices expensive and difficult to make.
The flexible, resilient top cover membrane 12 is rather delicate and thus prone to damage. A compromised or even damaged cover membrane would lead to erroneous pressure measurements and/or to leaking of the fluidic system, both of which are not acceptable. Consequently the top cover membrane 12 should be protected from mechanical damage as well as other detrimental environmental influences. At the same time the flexible top cover 12 has to remain accessible to the optical detection system 5.
The optical detection system 5 can be arranged within a suitable protective cover for the membrane 12. However, since a fluidic system of a liquid infusion pump system including any pressure sensor is generally designed as a disposable part, for hygienic reasons, such a solution is very expensive, since any light emitting and receiving devices of the detector system 5 would have to be discarded together with the micro-fluidic chamber 1.
It is important that the micro-fluidic chamber 1 of the pressure sensor device is free of air bubbles, in order to avoid systematic or random measurement errors. Air bubbles in the micro-fluidic sensor chamber (and more generally, anywhere within a fluidic system) reduce the stiffness of the fluidic system, and thus delay the response of the sensor to pressure changes that may occur if the fluidic system becomes occluded. The resulting irreproducible measurement errors can reduce the dosing accuracy of an infusion pump and increase the response time to an occlusion event.
Air bubbles present in a fluidic system of an infusion pumps, particularly in the pump system, but also in other components such as the container, also cause further problems. If air bubbles remains in the fluidic system, they may be administered instead of the liquid medication, which leads to potentially dangerous dosing errors. Furthermore, the administration of air into a patient should be generally avoided for medical reasons.
A further problem of fluidic systems, particularly in infusion pumps, is the dead volume in the fluidic system. The dead volume cannot be used, meaning that it cannot be emptied or drained completely. Thus, the dead volume considerably increases the effective cost per dose and thus of the overall therapy cost, since a certain percentage of the liquid medication inevitably remains in the fluid system and has to be disposed. This negative cost effect is particularly important for expensive medications. Furthermore, the relative portion of a certain dead volume of a given overall reservoir size increases with decreasing absolute reservoir size. Minimizing the dead volume therefore becomes more and more important with decreasing reservoir size.
To avoid air bubbles in the micro-fluidic chamber when the fluidic system is filled the first time, the so-called priming of the system, the chamber has to be filled in a controlled manner. However, this goal may be impeded by an uncontrolled orientation of the micro-fluidic chamber in space during this first filling procedure, since the gravitation field leads to buoyancy forces that act on the air bubbles. Depending on the orientation and the design of the micro-fluidic chamber, air bubbles may be caught in certain areas of the chamber.
Due to the many problems that may be caused by air bubbles in a fluidic system, there is a need for reliable sensors for use in infusion pumps for liquid medication, such as a pressure sensor and/or an air bubble sensor, that are able to detect air bubbles in fluidic systems that has small dead volume, inexpensively manufactured, a simpler optical detection system, is insensitive to small variations in the assembly of its components and has sensitive parts protected against mechanical damage.